SYNTHESIS OF CsSnI3 BY A SOLUTION BASED METHOD

ABSTRACT

This invention discloses a solution based synthesis of cesium tin tri-iodide (CsSnI 3 ). More specifically, the CsSnI 3  is fabricated in an organic Perovskite precursor solvent. CsSnI 3  are ideally suited for a wide range of applications such as light emitting and photovoltaic devices.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to the formation of materials forphotovoltaic devices and more specifically to the synthesis of CsSnI₃ bysolution based method.

2. Description of the Prior Art

The current photovoltaic technologies can be classified by the different10 materials used for the light absorption in a solar cell. Thesematerials include amorphous and polycrystalline silicon, CdTe,CuIn_(x)Ga_(1-x)Se₂ (CIGS), GaAs, and photosensitive organic dyes. Atransformative technology may emerge when a new and better material isdiscovered for photovoltaic applications.

CsSnI₃ is a unique phase-change material that exhibits four polymorphs.The black polymorph of CsSnI₃ could be obtained through a phasetransition from the yellow polymorph CsSnI₃ by increasing itstemperature above 425 K. It was further demonstrated by differentialthermal analysis and X-ray diffraction that during the cooling of theblack CsSnI₃ from 450 K, its ideal cubic Perovskite structure (B-α)deformed to a tetragonal structure (B-β) at 426 K, and became anorthorhombic structure (B-γ) below 351 K. [1] The CsSnI₃ is unique incombining two generally contra-indicated properties, strongphotoluminescence (PL) and high electrical conductivity. [2, 3]

A need still exists in the industry for developing synthesis methods forCsSnI₃, especially in large scale. The successful implementation ofthese materials for various applications requires a detailedunderstanding of both their processing and materials properties.

At present, the synthesis of CsSnI₃ can be divided into solid-phasesintering and solution based methods. The solid-phase sintering methodneeds vacuum and high temperature which means high production costs. [1]For solution based method, K. Shum and Z. Chen offered a simple way tosynthesize CsSnI₃, but the final product is not pure (U.S. PublishedPatent Application No. 2012/0306053). Here, we provide a simple solutionbased method to synthesize substantially pure CsSnI₃.

SUMMARY OF THE INVENTION

This invention is directed to synthesizing cesium tin tri-iodide(CsSnI₃) by a solution based method.

According to one aspect of the invention one embodiment in accordancewith the invention is directed to a process of forming homogeneousCsSnI₃ in an organic Perovskite precursor solvent, comprising steps of:

(1) forming CsI solution from CsI powder;

(2) providing SnI₂;

(3) adding the SnI₂ into the CsI solution to form a mixture wherein themolar ratio of the SnI₂ and CsI in the mixture is approximately 1:1;

(4) heating the mixed solution at a temperature within the range of 50°C. to 250° C. until all the solvent is evaporated to form CsSnI₃ powder;and

(5) the process steps (1) to (4) are performed in a substantially inertenvironment.

The substantially inert environment may be created within a glove boxand comprises a protective gas, such as N₂, including water vapor andoxygen the content of each of which is under 1 ppm and the temperaturewhile heating the mixed solution ranges from about 50° C. to 250° C.

The homogeneous CsSnI₃ is formed by adding a SnI₂ solution into a CsIsolution to form a mixture, and stirring the mixture for 1 to 3 hours toinsure that the raw materials have fully reacted, and then the solutionis aged for 12 to 24 hours to form the homogeneous CsSnI₃ precursorsolution.

The CsI solution is about 25 mmol/L to 2 mol/L CsI solution by fullydissolving CsI powder (99.999% purity) in a solvent, and the SnI₂solution is about 25 mmol/L to 2 mol/L SnI₂ solution by fully dissolvingSnI₂ powder (99% purity) in a solvent.

The solvent for dissolving CsI powder (99.999% purity) is selected toserve as a Perovskite ligand to form coordination complexes, such asN,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixtures thereof.

In the aforementioned process the SnI₂ is in the form of a SnI₂solution.

The solvent for dissolving SnI₂ powder (99.999% purity) is selected toserve as a Perovskite ligand to form coordination complexes, such asN,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixtures thereof.

The SnI₂ is in the form of a powder.

Other halides may be used to practice the invention. For example, aprocess of forming homogeneous CsSnI_((3-n))X_(n) in an organicPerovskite precursor solvent, comprises the steps of:

(1) forming CsI solution from CsI powder;

(2) providing SnI₂ and SnX₂;

(3) adding the SnI₂ and SnX₂ into the CsI solution to form a mixturewherein the molar ratio of the raw materials is SnX₂:SnI₂:CsI=y:(1-y):1,where 0≦y≦1;

(4) heating the final mixed solution at a temperature within the rangeof 50° C. to 250° C. until all the solvent is evaporated to formCsSnI_((3-n))X_(n), wherein X is a halogen element selected from GroupVIIA of the periodic table consisting of fluorine (F), chlorine (Cl),bromine (Br), iodine (I) and astatine (At) and 0≦n≦3; and

the process steps (1) to (4) are performed in a substantially inertenvironment.

In the aforementioned process the substantially inert environment may becreated within a glove box and comprises a protective gas, such as N₂,including water vapor and oxygen content both under 1 ppm and thetemperature while heating the final mixed solution ranges from about 50°C. to 250° C.

The homogeneous CsSnI_((3-n))X_(n) is formed by adding a mixed solutionof SnI₂ and SnX₂ into a CsI solution to form a mixture, and stirring themixture for 1 to 3 hours to insure that the raw materials fully reacted,and then the solution is aged for 12 to 24 hours to form thehomogeneou_(s) CsSnI_((3-n))X_(n) precursor solution.

The CsI solution is about 25 mmol/L to 2 mol/L CsI solution by fullydissolving CsI powder (99.999% purity) in a solvent, and the mixedsolution of SnI₂ and SnX₂ is about 25 mmol/L to 2 mol/L by fullydissolving SnI₂ and SnX₂ powder (99% purity) in a solvent.

The solvent for dissolving CsI powder (99.999% purity) is selected toserve as a Perovskite ligand to form coordination complexes, such asN,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixtures thereof.

The SnI₂ and SnX₂ are in the form of a SnI₂ and SnX₂ solution.

The solvent for dissolving the SnI₂ and SnX₂ powder (99.999% purity) isselected to serve as a Perovskite ligand to form coordination complexes,such as N,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixturesthereof.

The the SnI₂ and SnX₂ are in the form of a powder.

In the process, the steps are preferably performed in a glove box underthe protection of N₂ gas and the molar ratio of the SnI₂ and CsI in themixture is essentially 1:1.

The CsI solution is made by fully dissolving CsI powder (99.999% purity)in a solvent selected from the Perovskite precursor solutions, such asN,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixtures thereof.The concentration of CsI solution is about 25 mmol/L to 500 mmol/L.

The SnI₂ solution is made by fully dissolving SnI₂ powder (99% purity)in a solvent selected from the Perovskite precursor solutions, such asN,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixtures thereof.The concentration of SnI₂ solution is about 25 mmol/L to 500 mmol/L.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate preferred embodiments of the presentinvention, and together with the description, serve to explain theprinciples of the invention, in which:

FIG. 1 shows the schematic diagram for the synthesis of CsSnI₃; and

FIG. 2 shows the (a) X-ray diffraction data (XRD) profile taken fromCsSnI₃ (concentrations of CsI and SnI₂ were both 50 mmol/L) and (b)standard XRD pdf card (43-1162) of black-γ phase of CsSnI₃.

DETAILED DESCRIPTION AND EXAMPLES

The CsSnI₃ exhibits outstanding optical, electrical, and ferroelectricproperties. These features make CsSnI₃ ideally suited for a wide rangeof applications such as light emitting and photovoltaic devices.

More specifically, CsSnI₃ is a promising material in the application ofsolar cells since CsSnI₃ was found to possess a direct band gap of 1.32eV at room temperature, right in the narrow region of optimal band gapsfor the Shockley-Queisser maximum efficiency limit of a solar cell.

An effective method to synthesize large domain size high qualityPerovskite semiconductor according to the present invention isdisclosed. More specifically, a solution based method to synthesizeCsSnI₃ is disclosed according to the present invention. The CsSnI₃ canbe fabricated in an organic Perovskite precursor solvent as shown inFIG. 1. This synthesis method of the CsSnI₃ further enhances thelikelihood of using CsSnI₃ as a new absorption material for solar cells.

Examples of procedures for synthesizing polycrystalline CsSnI₃ usingreaction raw materials are described below. Generally, a process offorming homogeneous CsSnI₃ in an organic Perovskite precursor solvent,comprises the steps of:

(1) forming CsI solution from CsI powder;

(2) providing SnI₂;

(3) adding the SnI₂ into the CsI solution to form a mixture wherein themolar ratio of the SnI₂ and CsI in the mixture is substantially 1:1;

(4) heating the mixed solution at a temperature within the range of 50°C. to 250° C. until all the solvent is evaporated to form CsSnI₃ powder;and

the process steps (1) to (4) are performed in a substantially inertenvironment.

The substantially inert environment may be created within a glove boxand comprises a protective gas, such as N₂, including water vapor andoxygen content both under 1 ppm and the temperature while heating themixed solution ranges from about 50° C. to 250° C.

The homogeneous CsSnI₃ is formed by adding a SnI₂ solution into a CsIsolution to form a mixture, and stirring the mixture for 1 to 3 hours toinsure that the raw materials have fully reacted, and then the solutionis aged for 12 to 24 hours to form the homogeneous CsSnI₃ precursorsolution.

The CsI solution is about 25 mmol/L to 2 mol/L CsI solution by fullydissolving CsI powder (99.999% purity) in a solvent, and the SnI₂solution is about 25 mmol/L to 2 mol/L SnI₂ solution by fully dissolvingSnI₂ powder (99% purity) in a solvent.

The solvent for dissolving CsI powder (99.999% purity) is selected toserve as a Perovskite ligand to form coordination complexes, such asN,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixtures thereof.

The SnI₂ is in the form of a SnI₂ solution.

The solvent for dissolving SnI₂ powder (99% purity) is selected to serveas a Perovskite ligand to form coordination complexes, such asN,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixtures thereof.

The SnI₂ is in the form of a powder.

Other halides may be used to practice the invention. More generally, aprocess of forming homogeneous CsSnI_((3-n))X_(n) in an organicPerovskite precursor solvent, comprises steps of:

(1) forming CsI solution from CsI powder;

(2) providing SnI₂ and SnX₂;

(3) adding the SnI₂ and SnX₂ into the CsI solution to form a mixturewherein the molar ratio of the raw materials is SnX₂:SnI₂:CsI=y:(1-y):1,where 0≦y≦1;

(4) heating the final mixed solution at a temperature within the rangeof 50° C. to 250° C. until all the solvent is evaporated to formCsSnI_((3-n))X_(n), wherein X is a halogen element selected from GroupVIIA of the periodic table consisting of fluorine (F), chlorine (Cl),bromine (Br), iodine (I) and astatine (At) and 0≦n≦3; and

the process steps (1) to (4) are performed in a substantially inertenvironment.

The substantially inert environment may be created within a glove boxand comprises a protective gas, such as N₂, including water vapor andoxygen content both under 1 ppm and the temperature while heating thefinal mixed solution ranges from about 50° C. to 250° C.

The homogeneous CsSnI_((3-n))X_(n) is formed by adding a mixed solutionof SnI₂ and SnX₂ into a CsI solution to form a mixture, and stirring themixture for 1 to 3 hours to insure that the raw materials fully reacted,and then the solution is aged for 12 to 24 hours to form the homogeneousCsSnI_((3-n))X_(n) precursor solution.

The CsI solution is about 25 mmol/L to 2 mol/L CsI solution by fullydissolving CsI powder (99.999% purity) in a solvent, and the mixedsolution of SnI₂ and SnX₂ is about 25 mmol/L to 2 mol/L by fullydissolving SnI₂ and SnX₂ powder (99% purity) in a solvent.

The solvent for dissolving CsI powder (99.999% purity) is selected toserve as Perovskite ligand to form coordination complexes, such asN,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixtures thereof.

The SnI₂ and SnX₂ are in the form of a SnI₂ and SnX₂ solution.

The solvent for dissolving the SnI₂ and SnX₂ powder (99.999% purity) isselected to serve as a Perovskite ligand to form coordination complexes,such as N,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixturesthereof.

The SnI₂ and SnX₂ are in the form of a powder.

The procedures of synthesizing polycrystalline CsSnI₃ using reaction rawmaterials have been described.

The reaction raw materials were milled and dissolved in a glove boxunder protect of N₂ gas.

The conditions in the glove box were: room temperature or temperature of298.15 K (or 25° C., 77 F); water vapor and oxygen content are bothunder 1 ppm; and an absolute pressure of 100 kPa (or 14.504 psi, 0.986atm).

WORKING EXAMPLES Example 1 Preparation of CsI Solution

Initially, 0.13 gram of CsI (99.999% purity) powder was added to 10 mLGBL. The CsI powder was fully dissolved in GBL. The CsI solution wasstirred for 30 minutes.

CsI solution was colorless and stable in glove box.

It would be apparent to one skilled in the art that CsI solutions couldbe made using any solvents in addition to those used in the examples.Examples of solvents that can be used include but are not limited toN,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixtures thereof.

The concentration range of the CsI solution was from about 25 mmol/L to500 mmol/L.

Example 2 Preparation of SnI₂ Solution

Initially, 0.186 gram of SnI₂ (99% purity) powder was added to 10 mLGBL. The SnI₂ powder was fully dissolved in GBL. The SnI₂ solution wasstirred for 30 minutes.

SnI₂ solution was yellow and stable in glove box.

It would be apparent to one skilled in the art that SnI₂ solutions couldbe made using any solvents in addition to those used in the examples.Examples of solvents that can be used include but are not limited to,DMF, GBL or mixtures thereof.

The concentration range of the SnI₂ solution was from about 25 mmol/L to500 mmol/L.

Example 3 Synthesis of CsSnI₃

A given amount of the prepared CsI solution was transferred to areaction vial first. SnI₂ solution or powder was then slowly added intothe vial. The concentrations range of CsI and SnI₂ were both in a rangeof 25 mmol/L to 500 mmol/L, and their molar ratio was 1:1.

The mixed solution was stirred for 12 to 24 hours, and a uniform andtransparent yellow CsSnI₃ solution was formed.

The homogeneous CsSnI₃ solution was dried until the solvent was allevaporated. The heating temperature ranged from about 100° C. to 200° C.Then the pure black CsSnI₃ powder with metallic luster was obtained asshown in FIG. 1. The chemical reaction for the mixed solution could bedescribed as the following:

CsI+SnI₂→CsSnI₃

The reaction was verified by identifying the end products of CsSnI₃using the X-ray diffraction (XRD) data.

FIG. 2 (a) shows the XRD data profile taken from CsSnI₃ (concentrationsof CsI and SnI₂ were both 50 mmol/L).

FIG. 2 (b) showed the standard XRD pdf card (43-1162) of black-γ phaseof CsSnI₃.

All the measured peaks were well matched to the black-γ phase of CsSnI₃.

In summary, CsSnI₃ was synthesized using the CsI and SnI₂ by solutionbased method.

A solution based method, was employed to fabricate CsSnI₃, especiallysuitable for solar cell applications. The polycrystalline quality wascharacterized by XRD data.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

APPENDIX

-   1. I. Chung, J-H Song, J. Im, J. Androulakis, C. D. Malliakas, H.    Li, A. J. Freeman, J. T. Kenney, and M. G. Kanatzidis, J. Am. Chem.    Soc., 2012, 134, 8579-8587.-   2. K. Yamada, T. Matsui, T. Tsuritani, T. Z. Okuda, Naturforsch. A:    Phys. Sci., 1990, 45, 307-312.-   3. K. Shum, Z. Chen, J. Qureshi, C. Yu, J. J. Wang, W.    Pfenninger, N. Vockic, J. Midgley, J. T. Kenney, Appl. Phys. Lett.,    2010, 96, 221903.

1. A process of forming CsSnI₃ powder, comprising the steps of: (a)forming a CsI solution by dissolving CsI powder of purity equal to99.999% in an organic solvent consisting of at least one ofN,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixtures thereof;(b) forming a SnI₂ solution by dissolving SnI₂ powder of purity equal to99% in an organic solvent consisting of at least one ofN,N-dimethylformamide (DMF), γ-butyrolactone (GBL) and mixtures thereof;(c) adding the SnI₂ solution into the CsI solution to form a compositesolution wherein the molar ratio of the SnI₂ and CsI in said compositesolution is approximately 1:1; (d) stirring said composite solution forat least one hour to obtain a homogeneous CsSnI₃ precursor solution; (e)aging said precursor solution at least for a predetermined time periodafter said stirring step; (f) heating said precursor solution followingsaid aging step at a temperature within the range of 50° C. to 250° C.until all the solvent is evaporated to form CsSnI₃ powder exhibiting anXRD diffraction peak pattern for CsSnI₃ corresponding to the standardXRD-PDF card (43-1162) for the B-gamma-CsSnI₃ phase without exhibitingan XRD diffraction peak pattern for Cs₂SnI₆; and (g) the process steps(a) to (f) are performed in a substantially inert environment includinga protective gas and water vapor and oxygen each at a level below 1 ppm.2. The process of claim 1, wherein said substantially inert environmentis created within a glove box.
 3. The process of claim (1), wherein saidprecursor solution is aged for 12 to 24 hours to form a homogeneousCsSnI₃ precursor solution.
 4. The process of claim 3, wherein the CsIsolution is about 25 mmol/L to 2 mol/L CsI solution and the SnI₂solution is about 25 mmol/L to 2 mol/L SnI₂ solution.
 5. The process ofclaim 1, wherein said CsI powder is dissolved in said organic solvent toform CsI coordination complexes.
 6. (canceled)
 7. The process of claim1, wherein said SnI₂ powder is dissolved in said organic solvent to formSnI₂ coordination complexes.
 8. (canceled)
 9. (canceled)
 10. (canceled)11. (canceled)
 12. (canceled)
 13. (canceled)
 14. (canceled) 15.(canceled)
 16. (canceled)
 17. (canceled)
 18. (canceled)
 19. The processof claim 2, wherein said protective gas is N₂.
 20. (canceled) 21.(canceled)
 22. (canceled)
 23. (canceled)
 24. (canceled)
 25. (canceled)